1. Field of the Invention
The present invention relates to phosphate optical glass having improved characteristics in dispersion, transmission and chemical durability. More particularly, the present invention is directed to P.sub.2 O.sub.5 --R.sub.2 O (alkali metal oxide) --TiO.sub.2 --WO.sub.3 system optical glass having optical constants lying in the area defined and enclosed by seven points, A, B, C, D, E, F and G as shown in the accompanying drawing.
2. Description of the Prior Art
Among various kinds of optical glass there are known flint glass and heavy flint glass which belong to the group of high dispersion range. Conventionally a SiO.sub.2 --R.sub.2 O--PbO system has been used for the preparation of flint or heavy flint glass. Although this type of glass is relatively high in dispersion, the dispersive power thereof is not yet so high as to be satisfactory (cf. marks " " in the accompanying drawing, FIG. 1). In addition, this type of optical glass is deficient in chemical durability.
At present there is an increasing demand for such optical glass of further elevated dispersion which enables the lens designers to design higher quality lenses or to obtain further broadened freedom in lens design. In other words, the lens designers are now looking for a higher dispersion optical glass, that is, an optical glass having a higher dispersive power when optical glasses having the same refractive index are compared.